Since 1965, studies on an organic electroluminescent (EL) device (hereinafter, simply referred to as an ‘organic EL device’) leading to blue electric light emission using an anthracene single crystal had been continuously conducted, and in 1987, an organic EL device having a two-layer laminated structure including a hole layer (NPB) and a light emitting layer (Alq3) was proposed by Tang. Since then, the organic EL device has been proposed in the form of a multilayer-laminated structure which imparts each characteristic and subdivided function, such as an organic layer which is responsible for injecting and transporting holes, an organic layer which is responsible for injecting and transporting electrons, and an organic layer which induces electroluminescence to occur due to the combination of holes and electrons in the device in order to implement high efficiency and long lifetime characteristics required for commercialization.
In the organic EL device, when voltage is applied between two electrodes, holes are injected into the organic material layer at the anode, and electrons are injected into the organic material layer at the cathode. When the injected holes and electrons meet each other, an exciton is formed, and when the exciton falls down to a bottom state, light is emitted. Materials included in the organic material layer may be classified into a light emitting material, a hole injection material, a hole transporting material, an electron transporting material, an electron injection material, and the like according to the function.
In the electron spins of the excitons formed by recombining electrons and holes, the singlet exciton and the triplet exciton are produced at a ratio of 25% and 75%, respectively. In this case, the organic EL device may be classified into a fluorescent EL device in which singlet excitons contribute to light emission and a phosphorescent EL device in which triplet excitons contribute to light emission, according to the type of electron spin of the excitons formed.
In the fluorescent EL device in which light is emitted by singlet excitons, it is impossible for the internal quantum efficiency to theoretically exceed 25% according to the production ratio, and the external quantum efficiency of 5% is accepted as the limitation.
In the phosphorescent EL device in which light is emitted by triplet excitons, when a metal complex compound including a transition metal heavy atom such as Jr and Pt is used as a phosphorescent dopant, the light emitting efficiency may be improved up to 4 times compared to the fluorescent electroluminescent device.
As described above, the phosphorescent EL device exhibits theoretically higher efficiency than that of the fluorescent EL device in terms of light emitting efficiency. However, unlike green or red phosphorescent devices, in blue phosphorescent devices, the level of development for the color purity of a dark blue color, a phosphorescent dopant with high efficiency, and a host with a wide energy gap has been so little that commercialization has not even started, and instead, a blue fluorescent device has been used in products.
As the performance of the organic EL device has been improved to the level of commercialization characteristics due to the introduction of a multilayer-laminated structure, it has been attempted to expand the application range of the organic EL device from the start of a radio display product for a vehicle since 1997 to a mobile information display device and a display device for TV.
Further, according to the recent trends of an increase in size and a high resolution in a display, there is a need for developing an organic EL display having high efficiency and a long lifetime. In particular, the high resolution in a display may be implemented when more pixels are formed in the same area. Due to the high resolution, the light emitting area of the organic EL pixels has decreased, thereby resulting in reduction in the lifetime of the device. This reduction in the lifetime of the device has become the most important technical problem that the organic EL device needs to overcome.
However, since the materials for the organic EL device in the related art have a low glass transition temperature and thus are very poor in thermal stability, the materials fail to reach a level which is satisfactory in terms of a lifetime of an organic EL device, and need to be improved even in terms of light emitting characteristics.